The invention relates to a multilayer printed circuit board, in particular, for high-frequency operation, comprising at least a first, outer, plane dielectric layer for accommodating interconnection paths of equal cross-section and components as well as further alternately provided metallic and dielectric layers for forming a reference earth and for the voltage supply to said interconnection paths and components via plated-through holes.
The interconnection method used to form complex circuits employs printed circuits constructed according to multilayer technology, which circuits are referred to as multilayer printed circuit boards. In the case of steep signal edges of, for example, digital signals which are to be transferred from one integrated circuit to another integrated circuit, the influence of the interconnection paths has to be taken into account when the propagation time in the interconnection path corresponds approximately to the rise time of the circuit. This causes serious pulse distortions, reflections and more or less damped oscillations.
These errors can be precluded by using interconnection paths having a defined characteristic impedance. Such interconnection paths are termed microstrip lines which can be obtained by manufacturing all interconnection paths on one side of a dielectric plate or layer on which, if necessary, components are mounted, and applying a continuous metal layer to the opposite side of the dielectric plate, which side is subsequently connected to earth. As a result thereof, aII interconnection paths become microstrip lines.
For this reason, in high-frequency solutions special cross-sectional geometries of the interconnection paths and printed circuit boards are used to obtain specific characteristic impedances or interconnection-path impedances. In the case of, for example, a rectangular cross-section of an interconnection path, the characteristic impedance is determined by its width and height as well as by the thickness and permittivity of the printed circuit board, i.e. ultimately by the dielectric constant of the dielectric printed circuit board. When the interconnection paths have a round cross-section. apart from the permittivity, the diameter of the cross-section only plays a part when the thickness of the dielectric plate considerably exceeds the diameter or the height of the interconnection paths. This is known from, for example, the textbooks "Integrierte Mikrowellenschaltungen -Elektrische Grundlagen . . . , R. K. Hoffmann, Springerverlag Berlin Heidelberg New York Tokyo 1983, page 142 ff." and "Impulse auf Leitungen, W. Hillberg, Oldenburg Verlag Munchen Wien, page 121 ff.".
A problem which occurs frequently in high-frequency circuits is that a large part of said circuits is constructed for a specific characteristic impedance, whereas some parts of the circuit require a different, often higher, characteristic impedance, or that, analogous to the compensation of power dividers in high-frequency engineering, branches must be adapted in a broad band and with low reflection.
When interconnection paths having a rectangular cross-section are used, most step changes in characteristic impedance can be realised by adapting the cross-sectional width while the cross-sectional height remains the same. The customarily used etching techniques do not present any difficulties.
The adaptation of the characteristic impedances does present difficulties, however, when interconnection paths having equal, for example, round cross-sections are used. Such multilayer printed circuit boards in which for the manufacture of interconnection paths discrete wires are provided by very accurate wiring machines are known from multiwire- and microwire-techniques as described in, for example, "High Density Discrete Wiring Offers A Solution To Chip Carrier Design, C.L. Lassen, M. M. Motazedi, ELECTRONIC PACKAGING AND PRODUCTION. January, 1983", "A Discrete-Wired Solution for High Speed Surface - Mount Packaging, T. J. Buck, ELECTRONIC PACKAGING AND PRODUCTION, June 1985" and in the product-information document "MICROWIRE.TM.- Interconnection Technology, PCK Technology Division 322 South Surface Road, Melville, N.Y. 11747". As in such wiring techniques the wire diameter is fixedly determined, changes in characteristic impedance can, in principle, only be obtained by changing the thickness of the dielectric plates because in conventional manufacturing methods the permittivity can generally not be varied.
According to a known method, the thickness of the dielectric plate is changed by subjecting the relevant area to a milling operation. However, this leads to an unevenly structured surface, which considerably complicates, in particular, the provision of the interconnection paths in an automatic process and leads to higher characteristic impedance values instead of lower values in the area in question.
Other known methods of adapting the characteristic impedance lie in the field of circuit technology. They do not actually influence the characteristic impedance of an interconnection path but compensate undesired errors by a suitable circuit design. For example, reflections always occur in reproducible connections to a high-speed bus. The bus can be terminated at its end portion in a reflection-free manner, but all other coupling points will cause undesirable reflections. This problem can be minimized by providing the bus line with a termination having the lowest possible impedance, as the coupling points cause the effective impedance to decrease. This, however, leads to drive problems and, for this reason, the bus is driven at a predetermined impedance and all coupling points are coupled at a high-impedance. Reflections at the end portion of the bus must be accepted and, since their position is known. have to be taken into consideration by suitably designing the circuit layout. Thus, measures in the field of the design of the circuit are suitable under certain conditions only.